This invention is directed to monolithic catalyst support structures having a sintered ceramic structural phase and a high surface area support phase disposed on or within the ceramic. The invention is particularly directed to catalyst support structures of this kind which are improved by using a mixture of alumina and silica as the high surface area phase.
Conventional ceramic monolithic catalyst supports consist of a ceramic support material with a coating of high surface material upon which the catalyst is actually deposited. In particular, the ceramic support is normally prepared by sintering a mold of clay or other ceramic material at a high temperature to impart density and strength. This procedure normally results in the ceramic's having a very low surface area, and consequently the ceramic is coated with another material having a higher surface area, as well as specific chemical characteristics, on which to actually deposit the catalyst. This procedure of depositing a high surface area "washcoat" on the low surface area ceramic wall is disclosed, for example in U.S. Pat. Nos. 2,742,437 and 3,824,196. The high surface area washcoats of these catalyst supports, however, often lose the surface area during their service lives because of the high temperatures, such as those of automotive exhaust gases, to which they are exposed.
Recently, monolithic supports in which the high surface area phase is incorporated within the ceramic structural phase itself have been developed. In these supports, (disclosed in co-pending commonly-assigned U.S. Ser. Nos. 712,875 and 712,886, both filed Mar. 18, 1985) the high surface area material is coextruded with the sinterable ceramic material in a single step, and remains as a discrete phase within the ceramic matrix after the monolith is fired. Catalyst supports of this kind eliminate several mechanical and physical problems of those having an exterior high surface area "washcoat", but could be susceptible to thermal degredation of the high surface area phase because of exposure to elevated service temperatures. Additionally, because it is coextruded with the ceramic material, the high surface area oxide phase is present during firing of the ceramic and is therefore exposed to the high temperatures involved in that process as well. It is therefore desirable to use an oxide support material that can retain high surface area despite exposure to elevated temperatures during firing or in service.
Alumina and silica mixtures have been used as the supporting material for catalysts. British Pat. No. 1,581,628 broadly discloses the use of alumina-silica mixtures as an external washcoat support for catalysts, but discloses no specific compositions or particular preparations. U.S. Pat. No. 4,129,522 discloses the preparation of hydrogel spheres of 27-75% by weight alumina; 75-25% by weight silica. These spheres are taught to be useful as catalyst supports, but neither preparation nor service temperatures above 760.degree. C. are disclosed, and the patent contains no disclosure of use in a monolithic support structure. U.S. Pat. No. 4,151,121 discloses the use of alumina-silica mixtures containing 5-40% by weight alumina, prepared by gelation methods, as a catalyst support phase for incorporation into a granulated catalyst support. U.S. Pat. No. 3,637,525 discloses a monolithic catalyst support in which an alumina-silica mixture is combined with boron phosphate to form a singular structural/supporting phase for the catalytic materials. The alumina/silica mixture is prepared by coprecipitation or cogelation of alumina and silica in a weight ratio of 1.5:1 to 9:1.
It is an object of the present invention to provide an improved monolithic catalyst support having a strong structural phase of a sintered ceramic material and a second phase of a high surface area support material integral therewith which resists thermal degredation in service. It is a further object of the present invention to provide a monolithic catalyst support of this kind in which the support phase retains high surface area even when subjected to elevated temperatures used in ceramic firing processes. These and other objects are met by the invention to be described.